High performance chip packaging and method

ABSTRACT

A chip carrier package that includes a cover plate attached to the stiffener by a reflowable bonding material is disclosed. Additionally, a thermally and electrically conductive bonding material between the cover plate and the chip itself may be included. Also a chip package including an alignment device to aid in properly aligning the cover plate on the stiffener is disclosed. Furthermore, a method of packaging a chip including providing a reflowable material between the cover plate and stiffener body for attaching the cover plate to the stiffener, and simultaneously attaching the cover plate to the stiffener with an attaching of the carrier to an electronic circuit board is disclosed.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates generally to semiconductor chippackaging and carriers. More particularly, the present invention relatesto a chip packaging with a cover plate that is held onto a stiffener byan interposed bonding material. The present invention also includes analignment device for the cover plate and a method of packaging a chip.

[0003] 2. Related Art

[0004] Heretofore, adhesives or mechanical structures have been used toattach cover plates to stiffeners on chip packaging. The use ofadhesives create a number of problems. For instance, the use ofadhesives provides poor tolerances because of uneven layering because ofa failure of the adhesives to evenly or uniformly coat surfaces of acover plate and stiffener. Precise tolerances are required because thepressures created by the adhesives being unevenly layered createspressures on the chip, stiffener and cover plate that can createcracking. Further potential for cracking is developed by the need torepeatedly heat the package. For example, heating is required to connectthe chip to the carrier, then again for connecting the stiffener andcover plate, and then again to connect the carrier to other circuitrylike a printed circuit board. Poor tolerances also can create large gapsbetween the cover plate and stiffener resulting in poor thermalperformance. To address the above problems, cover plates and stiffenersmust be very accurately sized. However, this requirement adds complexityto the manufacturing process, i.e., the need to accurately form theparts and exactly locate them for joining. The need to compensate forpoor tolerances also necessitates the addition of material and, hence,weight to the cover plate and the overall package.

[0005] The use of mechanical connectors encounters similar problems asto those outlined above. Another disadvantage is that mechanicalconnectors add more undesirable weight to the package.

[0006] For the above reasons, there is a need in the art to have a chippackage, and process of manufacture therefor, where a cover plate thatis more accurately attached to a stiffener to achieve lower weight andreduced potential for die crack initiation. Further, there is, asalways, a need to increase thermal performance.

SUMMARY OF THE INVENTION

[0007] In a general aspect in accordance with the present invention isprovided a chip package having a chip carrier, a stiffener attached tothe carrier, and a cover plate attached to the stiffener by a reflowablebonding material. The bonding material is preferably a solder material.This aspect provides a number of advantages. For instance, the need toprecisely size parts is greatly reduced in that the cover plate isattached to the stiffener in such a way that it self-centers itselfwithin the stiffener when the reflowable bonding material is heated. Asa result, the potential for chip cracking is reduced. Further, becausethe cover plate need not be so precisely sized, lower weight can beobtained for the cover plate and the overall package. There is also anincrease in thermal performance not exhibited in the related art devicesbecause of the better cover plate to stiffener connection.

[0008] The above described aspect may also advantageously include aninterface material attaching the cover plate and the chip itself. Thisfurther aids in thermal conductivity and allows for setting the coverplate at an advantageous predetermined electrical potential, e.g.,ground, if the interface material is electrically conductive.

[0009] In a second general aspect in accordance with the presentinvention is provided a chip carrier including: a stiffener, a coverplate to cover the stiffener, and an alignment device to aid in properlyaligning the cover plate on the stiffener. This aspect aids in properlyaligning the cover plate on the stiffener to prevent die (or chip)cracking and assure even dispersion of the bonding material.

[0010] In a third general aspect in accordance with the presentinvention is provided a method of packaging a chip, the packaging havinga carrier, a stiffener having an opening in which the chip is positionedon the carrier, and a cover plate to cover the opening, the methodcomprising the steps of: providing a reflowable material between thecover plate and stiffener for attaching the cover plate to thestiffener, and attaching the cover plate to the stiffener simultaneouslywith an attaching of the carrier to other circuitry. This aspectprovides a process of manufacturing a chip package with fewer heatingcycles and, thus, the potential for reduced die (or chip) crackinitiation in the package.

[0011] The foregoing and other features and advantages of the inventionwill be apparent from the following more particular description ofpreferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The preferred embodiments of this invention will be described indetail, with reference to the following figures, wherein likedesignations denote like elements, and wherein:

[0013]FIG. 1 shows an exploded cross-sectional side view of a chippackage in accordance with a first embodiment of the present invention;

[0014]FIG. 2A shows a cross-sectional side view of a chip package inaccordance with the first embodiment of the present invention;

[0015]FIG. 2B shows a cross-sectional side view of a chip package withan enlarged cover plate in accordance with the first embodiment of thepresent invention;

[0016]FIG. 3. shows an exploded cross-sectional side view of a chippackage in accordance with a second embodiment of the present invention;

[0017]FIG. 4 shows a cross-sectional side view of a chip package inaccordance with the second embodiment of the present invention;

[0018]FIG. 5 shows a side view of a cover plate in accordance with athird embodiment of the present invention;

[0019]FIG. 6 shows a plan view of a stiffener in accordance with thethird embodiment of the present invention; and

[0020]FIG. 7 shows a plan view of a chip package in accordance with thethird embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Although certain preferred embodiments of the present inventionwill be shown and described in detail, it should be understood thatvarious changes and modifications may be made without departing from thescope of the appended claims. The scope of the present invention will inno way be limited to the number of constituting components, thematerials thereof, the shapes thereof, the relative arrangement thereof,etc., and are disclosed simply as an example of the preferredembodiment.

[0022] Referring to the drawings, FIG. 1 shows an explodedcross-sectional side view of a chip package in accordance with a firstembodiment of the present invention. The chip package 10 generallyincludes a cover plate 20, a stiffener 30, a chip 40 and a carrier 50.The stiffener 30 includes a base section 32 with a chip opening 37provided therein. The opening 37 is provided to house a chip 40. Thebase section 32 may also include risers 34, 36 that aid in generallypositioning the cover plate 20 within the opening 37, the advantages ofwhich will be discussed below. The stiffener 30 is attached to a carrier50, in the form of a plate 52, and acts to rigidify the plate 52. Thechip 40 generally includes a circuitry portion 42 and a plurality ofconnectors 44, e.g., bumping or controlled collapse chip connecting(C4), extending therefrom for connection to the carrier 52. The carrier52 may also include circuitry (not shown) along a surface thereof towhich the chip 40 is connected and a ball grid array 54 (BGA) forconnection to other circuitry on a substrate 80, e.g., an electroniccircuit board.

[0023] The cover plate 20 is constructed to have a base portion 21 andmay also include a raised area or pedestal 22. The stiffener 30 andcover plate 20 are made of metals, e.g., copper, stainless steel,aluminum, etc. or alloys thereof. The base portion 21 of the cover plateis sized to cover the opening 37 so as to close the opening 37. If thepedestal 22 is used, the pedestal 22 is sized to fit into the opening 37to further close off the opening. However, it is important to note thatthe cover plate 20, as shown in FIG. 2B, need not include the raisedarea 22. Further, the cover plate may be altered in size as necessary.For instance, the cover plate 20 may cover not only the opening 37 butalso the entire stiffener 30, as shown in FIG. 2B. In this instance, thecover plate 20 may require alteration from that of FIG. 1. Inparticular, the cover plate 20 may require openings or grooves 23 toaccommodate the shape of the risers 34, 36, the benefits of which willbe discussed infra.

[0024] In accordance with the present invention, and as shown in FIGS.2A and 2B, the cover plate 20 is attached to the stiffener 30 by athermally conductive bonding or reflowable material 70, 72. The bondingmaterial 70, 72 may take a variety of forms but must be able to bond thecover plate 20 to the stiffener 30 so as to provide good thermalconductivity therebetween. For instance, an adhesive or low melt soldermay be used, with solder materials being most preferable. Anotherpossibility is low melt solder paste which has been screened on to oneof the components. In terms of more particular substances, an adhesivethat has been used is a silicone-based material manufactured by GeneralElectric, Co. and sold as GE 3281®. In terms of low melt solder or lowmelt solder paste, tin lead based materials have been used. The generalfeature of the .low melt solder. is that it be highly thermallyconductive and readily flowable (reflowable) when heated so as touniformly coat the surfaces between the stiffener 30 and cover plate 20when heated. General features of other materials, such as GE 3281®, arethermally conductive and conformable when assembled.

[0025] The reflowable bonding material 70, 72 may be initially locatedalong the un-raised portion 23 of the cover plate, if a raised area 22is provided, or along an edge 33 of the opening 37. However, in order toattach the cover plate 20 to the stiffener 30, the material 70, 72 mustbe heated to melt or cure the reflowable bonding material. This step iscompleted after the heating step to connect the chip 40 to the carrier52, and during or before connecting the carrier 52 to other circuitry,e.g., to a substrate 80 of, for example, an electronic circuit board.The chip 40 is connected to the carrier 52 by the plurality ofconnectors 44. The temperature for the process is preferably in therange of 180 to 240 degrees Centigrade.

[0026] A further advantage of the present invention is that as thereflowable bonding material 70, 72 melts and flows it allows the coverplate 20 to self-center within the opening 37. As a result, the bondingmaterial 70, 72 is more evenly layered around the cover plate and,hence, pressures which can create cracking in the chip 40 are reduced.

[0027] As an alternative, the cover plate 20, and in particular theraised area 22 if provided, may also be attached to the top of the chip40 by an interface material 74. This allows for more heat conductivityaway from the chip 40 and enhanced thermal performance overall. Further,if connection of the cover plate 20 to the chip 40 is chosen, theinterface material 74, and if desired reflowable bonding materials 70,72, may also be electrically conductive. The provision of electricallyconductive material 74 (and, if desired, material 70, 72) allows thecover plate to be set to an advantageous predetermined electricalpotential with regard to the chip 40. For instance, the cover plate maybe grounded thus providing an additional safeguard for the chip 40 fromsuch things as static shock. In terms of the type of interface materialused, the material may take a variety of forms. For instance, theinterface material 74 may be low melt solder paste, an adhesive such asthe silicone-based material discussed above, and thermal grease, e.g.,zinc oxide based material. If the interface material is to beelectrically conductive, then low melt solder paste, such as a tin leadbased material has been found to be preferable.

[0028] Referring to FIGS. 3 and 4, a second embodiment of the presentinvention in which the risers 34, 36 are removed. In this embodiment,when the materials 170, 172 and 174 are heated to melt, the cover plate120 more freely self-centers in the opening 137. As a result, the raisedarea 122 is centered within opening 137 such that an even layer ofmaterial exists between an inside portion 125 of the raised area 122 andan inner edge 139 of the opening 137. It is important to note as withthe first embodiment, that the particular size of the cover plate 120may be altered to cover more or less of the stiffener 130 as necessary.

[0029] FIGS. 5-7 show a third embodiment of the present invention. Inthis embodiment the stiffener and cover plate combination are providedwith an alignment device 290 that aids in properly aligning the coverplate 220 on the stiffener 232. The cover plate 220 may include theraised area 222 including bonding material 274 thereon to attach to thechip. The stiffener includes an opening 237 for housing the chip 240.

[0030] The alignment device 290 generally includes a plurality of posts238 extending from the stiffener and a plurality of matching recesses oropenings 224 in the cover plate 220. The cover plate 220 is attached tothe stiffener 232 by a plurality of reflowable bonding material rings ordoughnuts 276. The rings 276 may be provided either on the stiffener232, or the cover plate 220 if enough surface tension exists to hold therings 276 to the cover plate 220. In either position, the rings willeventually encircle or surround the posts 238 for proper attachment. Anadvantage of the rings 276 is that they may be preformed and thenlocated around the posts 238 as necessary.

[0031] When the package is heated, the rings 276 melt and attach thecover plate 220 to the stiffener 232. Once again, the rings 276 andinterface material 274 may be made from the materials as outlined aboveexcept that it may no longer be efficient to screen on low melt solderpaste around the posts 238 for connection of the cover plate 220 andstiffener 232. Further, as noted above, the materials 270, 272, 274 maybe electrically conductive.

[0032] The alignment device 290 allows for more precise positioning ofthe cover plate 220 relative to the stiffener 232 which aids in creatingbetter tolerances, the reduction of uneven layering and crackprevention. It is important to note, however, that particular geometryof posts 238 and openings 224 may be altered as the alignment device maytake a variety of forms. For instance, the posts 238 and openings 224may be located at the corners of the cover plate 220. Further, thereneed not be four posts 238 and openings 224. As shown in FIG. 2B, analignment device may not require any openings that extend through thecover plate 20. Further, the cover plate need not require any openings,as shown in FIG. 1, as the alignment device may be provided by theprovision of risers 34, 36 which direct the cover plate 20 to thedesired position.

[0033] In terms of the method in accordance with the present invention,the chip 40 to carrier 50 connection step is preferably performed priorto connection of the cover plate 20 to the stiffener 30 or the carrier50 to other circuitry. Once completed, in accordance with the presentinvention, the stiffener 30 and cover plate 20 connecting step ispreferably completed simultaneously with the connection of the carrier50 to other circuitry, e.g., on a substrate 80 of electronic circuitboard. Further, if the cover plate 20 is to be connected to the chip 40,then this step may also be provided simultaneously with the carrier 50and other circuitry connecting step. The ability to make theseconnections simultaneously reduces the number of heating steps requiredto form a chip package and, hence, the potential for die (or chip) crackinitiation.

[0034] While this invention has been described in conjunction with thespecific embodiments outlined above, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art. Accordingly, the preferred embodiments of theinvention as set forth above are intended to be illustrative, notlimiting. Various changes may be made without departing from the spiritand scope of the invention as defined in the following claims. Forexample, the cover plate could also be attached after chip bonding andencapsulation and prior to attachment of the carrier to the electroniccircuit board.

We claim:
 1. A chip package comprising: a chip carrier; a stiffenerattached to the carrier; and a cover plate attached to the stiffener bya reflowable bonding material.
 2. The chip carrier of claim 1 , whereinthe reflowable bonding material is low melt solder.
 3. The chip carrierof claim 2 , wherein the low melt solder is tin lead based.
 4. The chipcarrier of claim 1 , wherein the reflowable bonding material is anadhesive.
 5. The chip carrier of claim 4 , wherein the silicone basedmaterial is a silicone-based adhesive.
 6. The chip carrier of claim 1 ,wherein the stiffener includes an aperture within which a chip ispositioned.
 7. The chip carrier of claim 6 , wherein the cover plate isattached to the chip by an interface material.
 8. The chip carrier ofclaim 7 , wherein the interface material is electrically conductive. 9.The chip carrier of claim 8 , wherein the cover plate is at apredetermined electrical potential.
 10. The chip carrier of claim 8 ,wherein the interface material is chosen from low melt solder paste,adhesive, and thermal grease.
 11. The chip carrier of claim 7 , whereinthe cover plate includes a raised area that mates with the aperture andis coupled to the chip by the interface material.
 12. The chip carrierof claim 1 , further comprising an alignment device operatively coupledto the stiffener to aid in proper alignment of the cover plate thereon.13. The chip carrier of claim 12 , wherein the alignment device includesa plurality of posts extending from the stiffener and a plurality ofmatching openings in the cover plate.
 14. The chip carrier of claim 12 ,wherein the bonding material surrounds the posts.
 15. The chip carrierof claim 12 , wherein the bonding material is electrically conductive.16. The chip carrier of claim 15 , wherein the cover plate is set at apredetermined electrical potential.
 17. A chip carrier comprising: astiffener; a cover plate to cover the stiffener; and an alignment deviceto aid in properly aligning the cover plate on the stiffener.
 18. Thechip carrier of claim 17 , wherein the alignment device includes aplurality of posts extending from the stiffener and a plurality ofmatching openings in the cover plate.
 19. The chip carrier of claim 17 ,wherein the cover plate is coupled to the stiffener by a bondingmaterial.
 20. The chip carrier of claim 19 , wherein the bondingmaterial encircles each post.
 21. The chip carrier of claim 17 , whereinthe stiffener includes an aperture therein and the cover plate includesa raised area thereon that fits into the aperture.
 22. The chip carrierof claim 21 , further comprising a chip positioned within the apertureand attached to the raised area by an interface material.
 23. The chipcarrier of claim 22 , wherein the interface material is chosen from anadhesive, low melt solder paste, and thermal grease.
 24. The chipcarrier of claim 23 , wherein the low melt solder paste is tin leadbased.
 25. A method of packaging a chip, the packaging having a carrier,a stiffener having an opening in which the chip is positioned on thecarrier, and a cover plate to cover the opening, the method comprisingthe steps of: providing a reflowable material between the cover plateand stiffener for attaching the cover plate to the stiffener; andattaching the cover plate to the stiffener simultaneously with anattaching of the carrier to other circuitry.
 26. The method of claim 25, wherein the step of attaching the cover plate includes heating thepackaging.
 27. The method of claim 26 , wherein the step of heatingincludes heating the packaging to a temperature in a range of 180° C. to240° C.
 28. The method of claim 26 , wherein the step of providing areflowable material includes providing one of a low melt solder, anadhesive, and a screened on low melt solder paste.
 29. The method ofclaim 28 , wherein the cover plate includes a raised area that fitswithin the opening and the step of attaching the cover plate includesself-centering the cover plate in the opening.
 30. The method of claim25 , further comprising the step of providing an interface materialbetween the chip and cover plate.
 31. The method of claim 30 , whereinthe step of attaching the cover plate to the stiffener simultaneouslywith an attaching of the carrier to other circuitry further includessimultaneously attaching the cover plate to the chip.
 32. The method ofclaim 30 , wherein the interface material is one of an adhesive, lowmelt solder paste, and thermal grease.
 33. The method of claim 32 ,wherein the low melt solder paste is tin lead based.
 34. The method ofclaim 32 , wherein the adhesive is a silicone based material.
 35. Themethod of claim 30 , wherein the interface material is electricallyconductive.
 36. The method of claim 35 , further comprising the step ofproviding the cover plate at a predetermined electrical potential. 37.The method of claim 25 , further comprising the step of providing analignment device between the cover plate and stiffener.